Polarisation radiation

In EMIRS and SNIFTIRS measurements the "inactive" s-polarlsed radiation is prevented from reaching the detector and the relative intensities of the vibrational bands observed in the spectra from the remaining p-polarised radiation are used to deduce the orientation of adsorbed molecules. It should be pointed out, however, that vibrational coupling to adsorbate/adsorbent charge transfer (11) and also w electrochemically activated Stark effect (7,12,13) can lead to apparent violations of the surface selection rule which can invalidate simple deductions of orientation. 

The surface actlve/surface inactive difference between p-polarlsed/ s-polarised radiation has enabled an alternative modulation technique, polarisation modulation, to be developed (15,16). In electrochemical applications, it allows surface specificity to be achieved whilst working at fixed potential and without electrochemical modulation of the interface. It can be implemented either on EMIRS or on SNIFTIRS spectrometers and can be very valuable in dealing with electrochemically irreversible systems however, the achievable sensitivity falls well short of that obtained with electrochemical modulation. It should also be noted that its "surface specificity" is not truly surface but extends out into the electrolyte with decreasing specificity to about half a wavelength. 

The laboratory experiments do not expose the surface to polarised radiation and some authors have suggested that the polarisation of radiation due to the scatter... 

For the dichroic photolysis mechanism to be successful, amino acids must be synthesised and destroyed in an intense circularly polarised radiation field. Daylight shows little or no excess but recent observations at 2.2 /rm of the Orion reflection nebula OMC-1 shows polarisations in excess of 17 per cent, although... 

The spectmm from an undulator is very different, and numerous peaks result from interference effects within the undulator. When the electron acceleration is confined to the orbit plane and the emission angle very low, the radiation is strongly elliptically polarised and, in the orbit plane itself, it is to within a few per cent linearly polarised. Use of a sequence of permanent magnets with magnetisation arranged in a spiral sequence enables circularly polarised radiation to be extracted from such a helical undulator and this radiation is particularly important for magnetic studies. 

Figure 1.2. Schematic representation of plane-polarised radiation projected along the Y axis at three different instants of time. The solid arrows denote the amplitude of the electric field (E), and the dashed arrows denote the perpendicular magnetic field (/ ).

Although it is simplest to describe and represent graphically the example of plane polarised radiation, it is also instructive to consider the more general case [2], For propagation of the radiation along the Y axis, the electric field E can be decomposed into components along the Z and X axes. The electric field vector in the X/ plane is then given by... 

Plane-polarised radiation is obtained when the phase factor a is equal to 0 or n and E = E. When a = 0, Ex and Ez are in phase, whilst for a = n they are out-of-phase by n. The special case illustrated in figure 1.2 corresponds to E = 0. Other forms of polarisation can be obtained from equations (1.6). For elliptically-polarised radiation we set a = n/2 so that equations (1.6) become... 

An essential requirement is that the characteristic time, T2, for the decay of the macroscopic polarisation must be much longer than the time taken for the polarising radiation pulse to dissipate. This requirement is readily satisfied the pin-diode S2 is held closed until the pulsed radiation has dissipated, and is then opened to capture the coherent radiation emitted by the polarised gas, due to one or more rotational transitions producing spontaneous emission. If all is well, the emission is detected against a near-zero radiation background. 

When a single, polarised radiation field is propagating in the material, Equation (3.80) will take the form ... 

Fig. 4. Reflection of IR radiation at a surface, showing incident vectors for P- and S-polarised radiation, and the components of the P-polarised radiation at the surface.

Consider an IR beam incident on the surface at an angle (p with respect to the surface normal (Fig. 4). The incident radiation can be resolved into components parallel (S-polarised) and normal (P-polarised) to the incident plane. The S-polarised radiation only has a component (S) parallel to the surface (in the y direction). However the p-polarised radiation has components parallel or tangential (Pt) to the surface, and perpendicular (P ) to the surface. Each layer (vacuum (e =1), adsorbate (e) and substrate (es)) is characterised by an isotropic complex dielectric constant (e) which is defined as 8 = (n + ik), where n in the refractive index, and k is the absorption coefficient. The change in reflectivity (AR) resulting from the adsorbate layer of thickness d for S- and P-polarised radiation is usually expressed as a ratio to the reflectivity (AR/R) -... 

Fig. 5. AR/R calculated as a function of incident angle

Fig. 6 Model calculations (employing the Lorentzian ocillator approximation and a three layer optical model) of AR/R for RAIRS on semiconducting or insulating (isotropic and nonabsorbing) substrates (ss=3) for P- and S-polarised radiation. Calculations are shown for two values of incident angle (<( ), below and above (pe- The adsorbate layer is assumed isotropic (e x=E y=s z) with e /e=0.5, v=2100cm, y=5cm . The convention AR=R-R ds is used, so that positive resonances correspond to absorption bands, and negative values transmission bands.

Fig. 6 shows the resonance associated with the coupling of the S-polarised radiation to the y component of an oscillating dipole (S), and the coupling of P-polarised radiation to the x and z components through Pt and P ... 

This is shown in Fig. 8. The coupling of P polarised radiation to the normal and parallel components of the oscillator (the Tiu v(C-O) mode) results in the expected transmission and absorption bands respectively. The former is blue shifted, and the latter is red shifted. This can be understood simply by considering the dynamic dipole coupling lattice sum expected for aligned oscillators perpendicular and pareillel to the surface [59], represented schematically in the inset of Fig. 8. 

Since the parallel components of the dynamic dipole are active in RAIRS, it is possible to use the azimuthal dependence to obtain the orientation of the adsorbate at the surface. A similar technique has been applied to adsorbates on metals in HREELS measurements made off specular in order to observe parallel modes through impact or resonant scattering processes. This was first demonstrated for the Rh(CO)2 molecule on anisotropic TiO2(110) surface [72]. The results of this study also allow a test of the three layer model theory (Fig.5,6) as applied to S-polarised radiation. Fig. 11 shows the FT-RAIRS spectrum for 1/3 monolayer of Rh(CO)2 on Ti02(l 10) measured with P and S polarised radiation. 

The azimuthal dependence of the intensity of Vasym(C-O) in the P-polarised radiation shows a maximum at 9=90° indicating an alignment of the Rh(CO)2 in the <110> direction. Since the S and Pt fields are orthogonal, using S-polarised radiation at 9 = 0° Vasym(C-O) is not observed, but is observed at 9 = 90°. The two most likely adsorption geometries of the adsorbed gem-dicarbonyl are shown in Fig. 11, both with the C-0 bonds in a plane aligned in the <110> direction. 

The effects of using polarised radiation on the spectra measured after exposure of the crystal to ethene at 373K are shown in Figure 16. Polarisation parallel to the crystal c axis enhances the contribution of the CH3 symmetric and asymmetric modes to the spectrum relative to the unpolarised spectrum or that obtained with polarisation perpendicular to the c axis. Similar polarisation effects were found for crystals exposed to ethene at room temperature or 473K, regardless of which crystal face the infrared beam was incident upon. After heating... 

If the incidence angle of / -polarised radiation is equal to the Brewster angle aB, the reflectivity from the pure (i.e., slick-free) water surface is close to zero. As a consequence, the water surface appears to be dark. In the presence of a film-forming substance, however, the slick patches represent a different optical medium that gives rise to a measurable reflectivity, which in turn makes the slick domains visible by their lighter appearance. This effect can be recorded by a Charge-Coupled-Device [CCD]-camera. 

The experimental arrangement is sketched in fig. 4.8. Plane polarised radiation is incident on an absorption cell placed between two crossed po-larisers in a magnetic field. The direction of the magnetic field is parallel to the direction of propagation. It is helpful to consider the three classical geometries associated with a+, 7r and a polarisations. These are... 

Due to the difference between the absorption coefficients a+(i/) and a (v), we have an ellipticity angle for initially plane-polarised radiation Due to the difference between the refractive indices +(v) and (v), we have a rotation angle of plane-polarised radiation ... 

From equation (12.10), one finds that interference between the direct bremsstrahlung process and polarisation radiation results in an asymmetry of the giant resonance profile observed in fluorescence with electron excitation even when the corresponding photoabsorption profile is symmetrical. This is exactly analogous to the Fano resonances (chapter 6) bremsstrahlung plays the role of the continuum, while the resonant chan-... 

According to the correspondence principle, the quantum number m can change by 1, 0, —1 where, for the transition m->m, the light radiated is polarised parallel to the direction of the field and for thm transition m l->m it is circularly polarised about the directionofl the field. A decrease in m corresponds to a Larmor precessioiflni the positive sense in the classical theory, and therefore to positive circularly polarised radiation an increase of m corresponds to negative circularly polarised radiation. 

A very useful technique which has recently been developed for Fe work, although its application is more general, is the use of polarised radiation. It is convenient to discuss the subject here, but the reader is advised to refer to the appropriate sections in later chapters for more detailed discussion of the spectra of some of the materials mentioned. Polarisation of the emitted y-ray was first shown in 1960 [36], and can take place by the Stark and Zeeman effects already familiar in optical polarisation studies [37]. 

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